SONAR (Sound Navigation and Ranging) is a technique that uses sound propagation under water to navigate or to detect objects in or on the water. As is known in the art, there are two types of sonar: passive and active. Passive sonar seeks to detect an object target by listening for the sound emanating from the obejct being sought. Active sonar creates a pulse of sound, and then listens for reflections of the pulse from a target object. To determine the distance to the target, elapsed time from emission of a pulse to reception is measured. To determine the directional bearing, several hydrophones are used to measure the relative arrival time to each in a process called beam-forming
The ability of a system to detect desired sound, or ‘signal’, in the presence of interfering sound, (i.e. noise or reverberation) is generally referred to as its ‘Recognition Differential’ (RD). RD is defined as the Signal-to-Interference ratio at which the system can, with some specific probability (usually taken at 50%), detect the signal while not exceeding a specified probability of false alert. RD is usually expressed in Decibels (dB). The lower the RD, the better the system performs.
It has been shown that the longer the ‘look’ or processing time, the lower the RD (see for analysis Burdic, W. S., Underwater Acoustic System Analysis, Prentice Hall, Englewood Cliffs, N.J., 1984). Since passive sonar systems can always be in a listening or ‘receive’ mode, and generally seek target signature components that are continuously radiating, they are able use many minutes of processing time and thus able to achieve very low RDs. By contrast, conventional active sonar systems typically use short pulse type transmissions and their receive processing time is therefore limited to a very few seconds, or even only fractions of a second. Consequently, active sonar RD are generally 10 to 30 dB higher than those of passive systems; this is equivalent to one to three orders of magnitude in linear terms.
Conventional active sonars are typically limited to relatively short duration pulse type transmissions for a number of reasons. One reason is that in some environments unwanted reverberation builds up as transmission times increases; this is particularly relevant to limited bandwidth systems in shallow water environments. A more fundamental reason is that most active sonars systems cannot receive while they are transmitting. Often, this is because they use the same device, (called a ‘transducer’), to both transmit and receive, and transducers cannot do both at the same time. Such systems are necessarily ‘mono-static’, meaning that their transmissions and receptions take place at one location. (Note that the converse is not necessarily true; some mono-static systems use co-located but separate devices for transmission & reception.) Most generally however, the inability to receive while transmitting is because active sonar transmission levels are inevitably so much higher than the levels of the echoes being sought that the acoustic transmit level overloads, or at least effectively ‘jams’, the receiver being used. This is even true for most so-called bi- or multi-static systems, where transmitting & receiving are done by separate devices located some distance apart.
It would be desirable to have a sonar system that would permit substantially continuous stream of incoming data that would not be limited to highly direction transmitters or high frequencies, particularly in sonar systems dedicated to the detection of targets, such as submarines, which are seeking to escape detection.
In prior art, U.S. Pat. No. 5,150,335 by Hoffman describes a waveform generation and processing technique that could be used to resolve Doppler and range ambiguity using interrupted frequency modulation for continuously transmitting sonar. Hoffman, however, does not address linearity and rejection criteria and therefore fails to teach those elements necessary to effectively to detect echoes while transmitting.
Teel et al., on the other hand, address in U.S. Pat. No. 4,961,174 the need for acoustic isolation from the transmitter and receiver and does so with physical vertical separation requiring a strong acoustic layer not processing rejection as in this approach and is therefore limited to relatively few specific environments (e.g., deeper water applications) and can be avoided by an intelligently operated object such as a submarine. U.S. Pat. No. 6,128,249 by Sullivan discusses a method of continuously transmitting by using sequences of tones each separated sufficiently in frequency to avoid interference. Sullivan like Hoffman does not address linearity and rejection criteria, and the series of tones used by Sullivan fail to permit effective range resolution and reverberation rejection.
In sum, the art fails to show how to effectively discriminate between signals that sonar receives from its own transmitter and echoes from the intended target subject to certain linearity and rejection requirements.